The National Institute of Standards and Technology's lead investigator, Dr Shyam Sunder, spoke to BBC Two's "The Conspiracy Files":

"Our working hypothesis now actually suggests that it was normal building fires that were growing and spreading throughout the multiple floors that may have caused the ultimate collapse of the buildings."

'Smoking gun'

However, a group of architects, engineers and scientists say the official explanation that fires caused the collapse is impossible. Architects and Engineers for 9/11 Truth argue there must have been a controlled demolition.

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The Conspiracy Files: 9/11 - The Third Tower is on BBC Two on Sunday 6 July at 2100 BST

The founder of the group, Richard Gage, says the collapse of the third tower is an obvious example of a controlled demolition using explosives.

"Building Seven is the smoking gun of 9/11. A sixth grader can look at this building falling at virtually freefall speed, symmetrically and smoothly, and see that it is not a natural process.

"Buildings that fall in natural processes fall to the path of least resistance", says Gage, "they don't go straight down through themselves."

Conspiracy theories

There are a number of facts that have encouraged conspiracy theories about Tower Seven.

Although its collapse potentially made architectural history, all of the thousands of tonnes of steel from the skyscraper were taken away to be melted down.

The third tower was occupied by the Secret Service, the CIA, the Department of Defense and the Office of Emergency Management, which would co-ordinate any response to a disaster or a terrorist attack.

The destruction of the third tower was never mentioned in the 9/11 Commission Report. The first official inquiry into Tower Seven by the Federal Emergency Management Agency was unable to be definitive about what caused its collapse.

In May 2002 FEMA concluded that the building collapsed because intense fires had burned for hours, fed by thousands of gallons of diesel stored in the building. But it said this had "only a low probability of occurrence" and more work was needed.

But now nearly seven years after 9/11 the definitive official explanation of what happened to Tower Seven is finally about to be published in America.

The National Institute of Standards and Technology has spent more than two years investigating Tower Seven but lead investigator Dr Shyam Sunder rejects criticism that it has been slow.

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The collapse of Tower 7

"We've been at this for a little over two years and doing a two or two and a half year investigation is not at all unusual. That's the same kind of time frame that takes place when we do aeroplane crash investigations, it takes a few years."

With no steel from Tower 7 to study, investigators have instead made four extremely complex computer models worked out to the finest detail. They're confident their approach can now provide the answers. Dr Sunder says the investigation is moving as fast as possible.

"It's a very complex problem. It requires a level of fidelity in the modelling and rigour in the analysis that has never been done before."

Other skyscrapers haven't fully collapsed before because of fire. But NIST argues that what happened on 9/11 was unique.

Steel structure weakened

It says Tower Seven had an unusual design, built over an electricity substation and a subway; there were many fires that burnt for hours; and crucially, fire fighters could not fight the fires in Tower 7, because they didn't have enough water and focused on saving lives.

Investigators have focused on the east side where the long floor spans were under most stress.

They think fires burnt long enough to weaken and break many of the connections that held the steel structure together.

Most susceptible were the thinner floor beams which required less fireproofing, and the connections between the beams and the columns. As they heated up the connections failed and the beams sagged and failed, investigators say.

The collapse of the first of the Twin Towers does not seem to have caused any serious damage to Tower Seven, but the second collapse of the 1,368ft (417m) North Tower threw debris at Tower Seven, just 350ft (106m) away.

Tower Seven came down at 5.21pm. Until now most of the photographs have been of the three sides of the building that did not show much obvious physical damage. Now new photos of the south side of the building, which crucially faced the North Tower, show that whole side damaged and engulfed in smoke.

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